In a wiring board (also referred to as “package”) with a built-in millimeter wave antenna for an IC including a circuit for millimeter wave band (wavelength: 1 mm to 10 mm and frequency: 30 GHz to 300 GHz), considerations should be made on several points in terms of the structure in order to secure required characteristics (high frequency transmission characteristics, antenna directivity and the like) when designing the structure of the package.
As one of the points, an IC (semiconductor chip) is mounted on a package by bonding using wires. Here, where the length (wire length) of each of the wires connecting electrode pads of the chip to the pads (each being a portion defined in the wiring) on the board is long, parasitic inductance increases and thus causes deterioration in the quality of signals. Thus, the chip needs to be arranged in such a way that the wire length can be made as short as possible. For this reason, a recessed portion (cavity) is formed in the package, and the chip is mounted in the cavity.
Here, the chip is mounted in such a way that the back surface (surface opposite to the surface where the electrode pads are formed) of the chip is adhered to the bottom surface of the cavity through an adhesive (electrically conductive paste or the like) applied to the bottom surface of the cavity. In addition, in order to make the wire length as short as possible, the cavity is formed to have a depth approximately equal to the thickness of the chip.
Furthermore, since the temperature of the IC (chip) including a millimeter wave circuit becomes high due to a high frequency operation of the chip, some measures to radiate the heat generated during the operation of the chip to the outside of the package need to be taken in order to prevent thermal runaway, destruction or the like of the chip. As a typical measure, a thermal via (via hole for heat radiation) is formed. This thermal via is formed to penetrate the board in the thickness direction thereof in a manner connecting the bottom surface (which is thermally bonded to the chip with an adhesive interposed therebetween) of the cavity provided in the package to the outside of the board.
As an example of the technique related to the conventional art, there is a semiconductor device described in Patent document 1 (International Publication Pamphlet No. WO 2003/023843). In this semiconductor device, a semiconductor chip is fixed onto a bottom surface of a quadrangular recess (cavity) provided in a wiring board, while one side of the chip is brought into contact with one side of the inner side wall of the cavity (FIG. 5). In addition, as another art, there is a semiconductor device described in Patent document 2 (Japanese Laid-open Patent Publication No. 9-283544). In this semiconductor device, a cavity is provided in a package, and grooves are formed on a surface of a cavity where the cavity and a semiconductor are to be in contact with each other. Moreover, as an example of the technique related to the heat radiation of a package, there is a technique described in Patent document 3 (Japanese Laid-open Patent Publication No. 2002-198660). In this technique, a circuit board is provided with multiple thermal via holes being arranged inside an insulating substrate and penetrating the substrate in the thickness direction thereof. Here, the thermal via holes are formed using a paste containing two types of Ag powders having different average grain sizes.
As described above, a package (wiring board) including a semiconductor element (chip) and transmission lines for high frequency waves such as millimeter wave bands is configured as follows for the purpose of securing required characteristics. A cavity is provided in a board and the chip is mounted in the cavity. In addition, the chip and a wiring portion formed on the board are electrically connected to each other by wire bonding. In other words, the chip is arranged in a manner that the wire length is made as short as possible. Thus, an increase in parasitic inductance is prevented.
Such a consideration in design can be expected to be somewhat effective for transmission of a high frequency (20 GHz, 2 GHz or the like) signal with a longer wavelength than a millimeter wave band (frequency: 30 GHz to 300 GHz), but is not so effective for transmission of a millimeter wave signal. Specifically, signal quality in millimeter wave signal transmission is deteriorated by factors including reflection caused by a difference in characteristic impedance between wiring paths, and minute parasitic inductance generated due to wiring, which has been ignorable heretofore. Accordingly, there is a need for taking measures such as making the length of a wire used in high frequency transmission paths, particularly ones used in millimeter wave transmission paths, as short as possible.
In addition, in the state of the art, a product using a frequency of 60 GHz or higher in the millimeter wave band has not yet been developed. Thus, a design taking various points such as a board configuration or a mounting method into consideration is needed.
Note that, in the configuration disclosed in Patent document 1 described above, the chip and the inner side wall are in close contact with each other in the cavity. For this reason, the heat released from the chip is likely to remain near this portion. Accordingly, the heat radiation properties near this portion are expected to be deteriorated as compared with the other region. Patent document 1, however, does not mention anything about a technique or a method for performing heat radiation at all.